**Author details**

Muhammad Nawaz National University of Singapore, Singapore

\*Address all correspondence to: geomn@nus.edu.sg

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Earth Crust*

**5.2 Exogenous factors**

**6. Summary and conclusion**

Another example of the evolution of the crust due to endogenous processes is volcanism, where material from the mantle or the deep crust is deposited onto the surface where it contributes in renewing the crust surface with new igneous rock and landforms. In some places the crust is weaker such as along plate boundaries, the magma forces its way through the rock, extruding rock and releasing pressure, which is why volcanic activity tends to occur near the borders of tectonic plates, for example, the Pacific Ring of Fire [22]. The composition and origin of the lava determine the type of volcanic landform created, with more fluid mafic lava forming structures such as shield volcanoes and more viscous felsic lava forming structures such as stratovolcanoes from the accumulation of ejecta. However, in cases where magma does not breach the surface, the magma in horns or magma chambers may solidify to form intrusive or plutonic rocks. Over time, the surrounding softer rock erodes away, revealing the harder plutonic rock beneath, which creates landforms

such as plutons, batholiths, dykes, sills, laccoliths and volcanic necks.

impact structure as opposed to a simple bowl shape [25].

The evolutionary processes mentioned above were all a result of forces originating from within the Earth. However, the crust is also shaped by a multitude of processes from external forces such as climate and extraterrestrial material. An overt example of an extraterrestrial force on the crust would be an impact crater, in which materials from space such as asteroids, meteoroids or comets collide with the Earth, leaving scars on the surface. While fairly infrequent in recent geological time, impacts were a major force of change during the late heavy bombardment period of the Earth's history [24], as the orbital path of the planet had not been fully cleared. The size of the impactor and extension diameter of the resultant impact crater is a decisive factor on the type of crater formed, with crater diameters above 2 km for sedimentary rocks and 4 km for crystalline rocks having a more complex

Climate and weathering are also significant drivers in the continued evolution of the crust. And while the parameters that control climate are complex and not fully understood, its effects can be seen widely. These processes can be observed in many forms, such as the exposure of batholiths by the erosion of soft rock, the carving of the Grand Canyon or the deposition of sediment by fluvial processes to create river deltas [26]. Additionally, biological processes also play a role in weathering and erosion. For example, plant roots hold the soil together, providing resistance to erosion [25]. Plants and burrowing animals also contribute to the mechanical breakdown of rock through wedging caused by growth and burrowing, respectively. All of the above processes are but a fraction of the factors that keep the Earth's crust in a state of constant flux. And while we may be unable to observe all geological evolutionary phenomena in the span of a human lifetime, we have more than enough examples

While the crust may only comprise the superficial layer of the Earth, it is truly a dynamic and fascinating thing to learn about. Superficially appearing to be a solid immutable covering of rock on our world, it is actually a collection of gargantuan rock plates of heterogeneous composition floating upon an equally colossal ocean of magma that is the outer mantle. From its early origins as a hot lifeless shell covering our planet to its current state as the home for all life on Earth, it has changed so much over the geological timespan of the Earth's history and continues to evolve to

and evidence to show that truly drastic changes occur in geological time.

**6**
